US 7102823 B2
A diffractive security element (2) is divided into surface portions, having an optically effective structure (9) at interfaces embedded between two layers of a layer composite (1) of plastic material. At least the base layer (4), which is to be illuminated, of the layer composite (1) is transparent. The optically effective structure (9) as a base structure has a zero order diffraction grating with a period length of at most 500 nm. In at least one of the surface portions an integrated optical waveguide (5) with a layer thickness (s) of a transparent dielectric is embedded between the base layer (4) and an adhesive layer (7) of the layer composite (1) and/or a protective layer (6) of the layer composite (1), wherein the profile depth of the optically effective structure (9) is in a predetermined relationship with the layer thickness (s). Upon illumination with white incident light (13) the security element (2) produces light (14) which is diffracted in the zero diffraction order, of high intensity and with an intensive color.
1. A diffractive security element having an optical waveguide comprising a transparent dielectric integrated into a layer composite and embedded between a transparent base layer to be illuminated and a protective layer, wherein the dielectric differs in refractive index from the plastic material of the adjoining layers and in surface portions bears closely against an optically effective structure of an interface in relation to the base layer,
in the waveguide the transparent dielectric is of uniform layer thickness (s) and is of a value of the refractive index of at least 2,
the waveguide is modulated by means of the optically effective structures and the optically effective structure as a base structure has a zero order diffraction grating with a diffraction grating vector, a period length (d) from the range of between 100 and 500 nm and a profile depth (t) from the range of between 20 nm and 1 μm,
the waveguide is of a minimum length (L) of at least between 10 and 20 period lengths (d) of the zero order diffraction grating, and
in at least one of the surface portions, the profile depth (t) and the layer thickness (s) for modulation of the waveguide are in a predetermined ratio of either t≈3s, s≈t or s≈2t.
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This application claims priority based on PCT Application No. PCT/EP02/12243, filed on Nov. 2, 2002 and Swiss Patent Application No. 0084/02, filed on Jan. 18, 2002, both of which are incorporated herein by reference in their entirety.
The invention relates to a diffractive security element which is divided into surface portions with an optically effective structure of interfaces embedded between two layers of a composite of plastic material.
Diffractive security elements of that kind are used for the verification of articles such as banknotes, passes and identity cards of all kinds, valuable documents and so forth in order to be able to establish the authenticity of the article without involving a high level of cost. When the article is issued the diffractive security element is fixedly joined thereto, in the form of a stamp portion cut from a thin layer composite.
Diffractive security elements of the kind set forth in the opening part of this specification are known from EP 0 105 099 A1 and EP 0 375 833 A1. Those security elements include a pattern of surface elements which are arranged in a mosaic-like fashion and which have a diffraction grating. The diffraction gratings are azimuthally predetermined in such a way that, upon a rotary movement, the visible pattern produced by diffracted light performs a predetermined sequence of movements.
U.S. Pat. No. 4,856,857 describes the structure of transparent security elements with impressed microscopically fine relief structures. Those diffractive security elements generally comprise a portion of a thin layer composite of plastic material. The interface between two of the layers has microscopically fine reliefs of light-diffracting structures. To enhance reflectivity the interface between the two layers is covered with a mostly metallic reflection layer. The structure of the thin layer composite and the materials which can be used for that purpose are described for example in U.S. Pat. No. 4,856,857 and WO 99/47983. It is known from DE 33 08 831 A1 for the thin layer composite to be applied to an article by means of a carrier film.
The disadvantage of the known diffractive security elements lies in the difficulty of visually recognising complicated, optically varying patterns in a narrow solid angle and the extremely high level of surface brightness, at which a surface element occupied by a diffraction grating is visible to an observer. The high level of surface brightness can also make it difficult to recognise the shape of the surface element.
A security element which is simple to recognise is known from WO 83/00395. It comprises a diffractive subtractive color filter which, upon illumination with for example daylight, in a viewing direction, reflects red light and, after rotation of the security element in the plane thereof through 90°, reflects light of another color. The security element comprises fine bars, embedded in plastic material, the bars being of a transparent dielectric with a refractive index which is much greater than that of the plastic material. The bars form a grating structure with a spatial frequency of 2500 lines/mm and reflect in the zero diffraction order red light with a very high level of efficiency if the white light incident on the bar structure is polarised in such a way that the E-vector of the incident light is oriented in parallel relationship with the bars. For spatial frequencies of 3100 lines/mm the bar structure reflects green light in the zero diffraction order, while for even higher spatial frequencies the reflected color goes into the blue range in the spectrum. According to van Renesse, Optical Document Security, 2nd Edition, pages 274–277, ISBN 0-89006-982-4, such structures are difficult to produce inexpensively in large amounts.
U.S. Pat. No. 4,426,130 describes transparent, reflecting sinusoidal phase grating structures. The phase grating structures are so designed that they have the highest possible level of diffraction efficiency in one of the two first diffraction orders.
The object of the present invention is to provide an inexpensive diffractive security element which is simple to recognise and which in daylight can be easily visually checked.
The specified object is attained in accordance with the invention by the features recited in the characterising portion of claim 1. Advantageous configurations of the invention are set forth in the appendant claims.
Embodiments by way of example of the invention are described in greater detail hereinafter and are illustrated in the drawing in which:
In another embodiment of the security element in which transparency is not required the protective layer 6 and/or the adhesive layer 7 is colored or black. A further configuration of the security element only has the protective layer 6 if that embodiment is not intended for being stuck on.
The layer composite 1 is produced for example in the form of a plastic laminate in the form of a long film web with a plurality of mutually juxtaposed copies of the security element 2. The security elements 2 are for example cut out of the film web and joined to the substrate 3 by means of the adhesive layer 7. The substrate 3, mostly in the form of a document, a banknote, a bank card, a pass or identity card or another important or valuable article, is provided with the security element 2 in order to verify the authenticity of the article.
So that the waveguide 5 is optically effective the waveguide 5 comprises a transparent dielectric, the refractive index of which is considerably higher than those of the plastic materials for the base layer 4, the protective layer 6 and the adhesive layer 7. Suitable dielectric materials are set out for example in above-mentioned specifications WO 99/47983 and U.S. Pat. No. 4,856,857, Tables 1 and 6. Preferred dielectrics are ZnS, TiO2 and so forth with refractive indices of n≈2.3.
The waveguide 5 fits closely to the interface relative to the shaping layer 11, which has the optically effective structure 9, and is therefore modulated with the optically effective structure 9. The optically effective structure 9 is a diffraction grating with such a high spatial frequency f that the light incident 13 at an angle of incidence α relative to the surface normal 12 of the security element 2 is diffracted by the security element 2 only into the zero diffraction order and the diffracted light 14 is reflected at the angle of reflection β, wherein: angle of incidence α=angle of reflection β. This establishes for the spatial frequency f a lower limit of about 2200 lines/mm and an upper limit for a period length d of 450 nm. Those diffraction gratings are referred to as ‘zero order diffraction gratings’ and are meant by ‘diffraction grating’. In the drawing in
The waveguide 5 begins to perform its function, that is to say to influence the reflected light 14, if the waveguide 5 includes between at least 10 and 20 periods of the optically effective structure 9 and therefore has a minimum length L, dependent on the period length d, of L>10d. Preferably the lower limit of the length L of the waveguide 5 is in the range of between 50 and 100 period lengths d so that the waveguide 5 affords its optimum effectiveness.
In an embodiment the security element 2, over its entire area, has a uniform diffraction grating for the optically effective structure 9 and a waveguide 5 of uniform layer thickness s. In another embodiment surface portions arranged in a mosaic configuration form an optically easily recognisable pattern. So that a surface portion of the mosaic can be recognised by an observer using the naked eye, in its contours, the dimensions are to be selected to be larger than 0.3 mm, that is to say at any event the waveguide 5 is of a sufficient minimum length L.
The security element 2 which is illuminated with white diffuse incident light 13 changes the color of the reflected diffracted light 14 if its orientation relative to the viewing direction is altered by means of a tilting or rotary movement. The axis of rotation of the rotary movement is the surface normal 12 while the tilting movement takes place about an axis of rotation which is in the plane of the security element 2.
The zero order diffraction gratings exhibit a pronounced behaviour in relation to polarised light 13, which is dependent on the azimuthal orientation of the diffraction grating. For describing the optical properties involved, in
For example the light beam BnTM is incident in the diffraction plane 16 perpendicularly on to the grating lines of the security element 2, with polarisation of the electrical field in the diffraction plane 16.
Depending on the respective parameters of the optically effective structure 9 and the waveguide 5 (
In an embodiment of the security element 2 the values for the profile depth t of the optically effective structure 6 and the layer thickness s are approximately equal: that is to say s≈t, the waveguide 5 being modulated with the period d=370 nm. Preferably the layer thickness is s≅t=75±3 nm. If the light beam BnTE incident in the one diffraction plane 16 (
That behaviour on the part of the security element 2 does not change substantially, except for slight color shifts, if the layer thickness of the waveguide 5 is varied between 65 nm and 85 nm and the profile depth t between 60 nm and 90 nm.
A reduction in the period length d to 260 nm in other embodiments shifts the color of the diffracted light 14 with an incident light beam BnTE from green to red and with an incident light beam BpTM from red to green. The color red produced by the light beam BnTE changes to orange upon tilting of the security element 2 in the direction of smaller angles in the region of α=20°.
Another embodiment of the security element 2 exhibits an advantageous optical behaviour as, upon illumination with white unpolarised light 13, for small tilting angles, corresponding to the angle of incidence between α=10° and α=40°, the color of the diffracted light 14 remains practically invariant. The parameters of the waveguide 5, the layer thickness s and the profile depth t are here linked by the relationship s≈2t. For example the layer thickness s=115 nm and the profile depth t=65 nm. The period length d of the optically effective structure 9 is d=345 nm. In the specified range of the tilt angle with illumination with white unpolarised light 13 in parallel relationship with the grating lines of the optically effective structure 9 the diffracted light 14 is of a red color, to which the light beams BpTM primarily contribute. Upon a rotary movement of the security element 2 through a few degrees of azimuth angle the reflected color remains red while upon a further increasing rotary angle two colors are reflected symmetrically with respect to red, of which the shorter-wave color shifts in the direction of ultraviolet and the longer-wave color rapidly disappears in the infrared range. For example with an azimuth angle of 30° the shorter-wave color is an orange; the longer-wave color is invisible to the observer.
If the security element 2 is rotated in such a way that the incident light 13 is directed in perpendicular relationship to the grating lines, the security element 2 of Example 2, upon tilting about an axis in parallel relationship with the grating lines of the diffraction grating, exhibits a color shift: for example the observer views the surface of the security element 2 with perpendicular incidence of light, that is to say with an angle of incidence α=0°, as an orange, with an angle of incidence α=10° the observer sees a mixed color comprising about 67% green and 33% red and with an angle of incidence α=30° he sees an almost spectrally pure blue.
In another embodiment of the security element 2 the optically effective structure 9 comprises at least two mutually crossing diffraction gratings. The diffraction gratings advantageously cross at intersection angles in the range of between 10° and 30°. Each diffraction grating is determined for example by a profile depth t of 150 nm and a period length of d=417 nm. The layer thickness s of the waveguide 5 is s=60 nm so that the parameters s and t of the waveguide 5 satisfy the relationship t≈3s. Upon illumination with white, unpolarised incident light 13 in perpendicular relationship to the grating lines of the first diffraction grating, upon tilting about an axis parallel to the grating lines of the first diffraction grating, there is a color shift, for example from red to green or vice versa. That behaviour is maintained after a rotation through the angle of intersection as now the tilt axis is oriented in parallel relationship with the grating lines of the second diffraction grating.
In the further embodiment of the security element 2 which is shown in cross-section in
In accordance with the height of the relief profile 17 or a blaze angle γ of the sawtooth profile, upon illumination of the security element 2 by means of light 13 which is incident at the angle of incidence α measured with respect to the surface normal 12, the diffracted light 14 is reflected at a larger reflection angle β1. The incident light 13 is incident at the angle γ+α relative to the perpendicular 18 on to the plane of the waveguide 5, which is inclined by virtue of the relief profile 17, and is reflected in the form of diffracted light 14 at the same angle relative to the perpendicular 18. The reflection angle β1, related to the surface normal 12, is β1=2γ+α. The advantage of that arrangement is facilitated viewing of the optical effect produced by the security element 2. It is to be noted here that refraction in the materials of the layer composite 1 (
In quite general terms a high level of diffraction efficiency of almost 100% is typical of those security elements 2 (
The parameters in accordance with Table 1 can be used for those security elements 2.
The parameter period length d determines the color of the light 14 which is diffracted reflected into the zero order. A change in the parameter layer thickness s of the waveguide 5 (
After rotation of the substrate 3 with the stamp portion or tag 23 through an angle of 90°, as shown in
In another embodiment of the security element 2 the arrangement of a plurality of identical surface portions 21 on the stamp portion or tag 23 can form a circular ring, the diffraction grating vectors 19 being directed on to the center of the circular ring. With a viewing direction along a diameter of the circular ring, irrespective of the azimuthal position of the substrate 3, the most remote (0°±20°) and the closest (180°±20°) portions of the circular ring light up in a green color and the regions which are furthest away from the diameter at 90°±20° and 270°±20° respectively of the circular ring light up in a red color. Regions disposed therebetween exhibit the above-described mixed color comprising two adjacent spectral ranges. The color pattern is invariant with respect to a rotation of the substrate 3 and appears to move relative to any indicia 8 (
In a further configuration of
It will be appreciated that, without limitation, all the above-described embodiments of the security elements 2 can advantageously be combined as the specific optical effects which are dependent on the azimuth or the tilt angle, by virtue of the mutual referencing thereof, are substantially more striking and can therefore be more easily recognised.
Finally other embodiments of the security element 2 also have field portions 26 (